Earth-boring tools are used to form boreholes (e.g., wellbores) in subterranean formations. Such earth-boring tools include, for example, drill bits, reamers, mills, etc. For example, a fixed-cutter earth-boring rotary drill bit (often referred to as a “drag” bit) generally includes a plurality of cutting elements secured to a face of a bit body of the drill bit. The cutting elements are fixed in place when used to cut formation materials. A conventional fixed-cutter earth-boring rotary drill bit includes a bit body having generally radially projecting and longitudinally extending blades. During drilling operations, the drill bit is positioned at the bottom of a well borehole and rotated as weight-on-bit (WOB) is applied.
A plurality of cutting elements is positioned on each of the blades. The cutting elements commonly comprise a “table” of superabrasive material, such as mutually bound particles of polycrystalline diamond, formed on a supporting substrate of a hard material, such as cemented tungsten carbide. Such cutting elements are often referred to as “polycrystalline diamond compact” (PDC) cutting elements. The plurality of PDC cutting elements may be fixed within cutting element pockets formed in each of the blades (e.g., formed in rotationally leading surfaces of each of the blades). Conventionally, a bonding material, such as a braze alloy, may be used to secure the cutting elements to the bit body. One or more surfaces of the cutting table act as a cutting face of the cutting element. During a drilling operation, one or more portions of the cutting face are pressed into a subterranean formation. As the earth-boring tool moves (e.g., rotates) relative to the subterranean formation, the cutting table drags across surfaces of the subterranean formation and the cutting face removes (e.g., shears, cuts, gouges, crushes, etc.) a portion of formation material.
Rotary drill bits carrying such PDC cutting elements have proven very effective in achieving high rates of penetration in drilling subterranean formations exhibiting low to medium hardness. In harder subterranean formations, the WOB applied on a downhole tool, such as a PDC bit, and similarly the torque-on-bit (TOB) applied to the tool, are typically limited to protect the PDC cutting elements. In order to obtain higher rate-of-penetration (ROP) in hard subterranean formations, PDC bits may be used at increased rates of rotation (i.e., increased revolutions per minute (RPM)). At higher RPMs, however, the bit may become particularly prone to dynamic dysfunctions caused by instability of the bit, which may result in damage to the PDC cutting elements, the bit body, or both.
Adjustments may be made to the bit structure in order to increase drilling efficiency while reducing mechanical specific energy (MSE) (i.e., the amount of energy required to remove a given volume of rock). Improvements in stability of rotary drill bits have reduced prior, notable tendencies of such bits to vibrate in a deleterious manner. Several approaches to realizing drilling stability have been independently practiced on bits, including anti-whirl or high-imbalance designs, low-imbalance designs, and kerfing.
One approach for increasing stability involves configuring the rotary drill bit with a selected imbalance force configuration and is conventionally referred to as a so called “anti-whirl” bit. Bit “whirl” is a phenomenon wherein the bit precesses around the well bore and against the side wall in a direction counter to the direction in which the bit is being rotated. Whirl may result in a borehole of enlarged (over gauge) dimension and out of round shape and may also result in damage to the cutters and the drill bit. A so called anti-whirl design or high-imbalance concept typically endeavors to generate an imbalance force (i.e., the imbalance force being the summation of each of the drilling forces generated by each of the cutting elements disposed on a rotary drill bit) that is directed toward a gage pad or bearing pad that slidingly engages the wall of the borehole. Such a configuration may tend to stabilize a rotary drill bit as it progresses through a subterranean formation.
Various other methods and equipment have been proposed to enhance (e.g., magnify) the natural imbalance forces, including using dynamically balanced lower drillstring assemblies and realigning the cutters to enhance the imbalance forces.